Momentum spectrometer

ABSTRACT

The invention relates to a magnetic mass spectrometer of very high dispersion and high resolving power. At the same time, the technical expenditure involved, especially the weight of the magnets and the physical dimensions of the magnets and of the vacuum chamber, are kept extremely small. The spectrometer comprises a special magnetic dispersion field with merely dispersing but no focusing properties. The dispersion field is made up of several magnetic homogeneous fields with straight, parallel field boundaries for the entrance and the exit of ions, called &#39;&#39;&#39;&#39;magnetic prisms&#39;&#39;&#39;&#39; which are successively transversed by the ions. The homocentric beams of ions emanating from the ion source are converted by electric or magnetic lenses into parallel beams which are conducted to the dispersion field. After leaving the dispersion field, the parallel ion beams are focused again by electric or magnetic lenses and passed into the means for collecting the ions. The use of a larger number of &#39;&#39;&#39;&#39;magnetic prisms&#39;&#39;&#39;&#39; successively traversed by the ions as outlined above will generated any desired degree of dispersion.

United States Patent Kleinwiichter [541 MOMENTUM SPECTROMETER [72]v Inventor: Hans Kleinwiic hter, Kreuzstrasse 105, D-785 Lorrach/Baden, Germany 1 Y [22] Filed: May 25,1971

21 Appl. No.: 146,649

Related U.S. Application Data [63] Continuation-in-part of Ser. No. 15,311, March 4, 1970, abandoned, which is a continuation of Ser. No. 551,790, May 20, 1966, abandoned.

[30] Foreign Application Priority Data 1151 3,686,500 1 1 Aug. 22, 1972 Primary Examiner-William F. Lindquist Attorney-Jacob L. Kollin ABSTRACT The invention relates to a magnetic'mass spectrometer of very high dispersion and high resolving power. At the same time, the technical expenditure involved,

especially the weight of the magnets and the physical dimensions of the magnets and of the vacuum chamber, are kept extremely small. The spectrometer comprises a special magnetic dispersion field with merely dispersing .but no focusing properties. The

dispersion field is madev up of several magnetic homogeneous fields with straight, parallel field boundaries for the entrance and the exit of ions, called magnetic prisms which are successively transversed Y by the ions. The homocentric beams of ions emanating from the ion source are converted by electric or magnetic lenses into parallel beams which are conducted to the dispersion field. After-leaving the dispersion field, the parallel ion beams are focused again by electric or magnetic lenses and passed into the means for collecting the ions. The use of a larger number of magnetic prisms successively traversed by the ions as outlined above will generated any desired degree of dispersion.

11 Claims, 17 Drawing Figures Patented Aug. 22, 1972 3,686,500

8 Sheets-Sheet 1 Patented Aug. 22, 1972 3,686,500

8 Sheets-Sheet 2 Patented Aug. 22, 1972 8 Sheets-Sheet 5 Patented Aug 22, 1972 3,686,500

8 Sheets-Sheet 4 Patented Aug. 22, 1972 8 Sheets-Sheet 6 Patented Aug. 22, 1972 3,686,500

8 Sheets-Sheet 7 Patented Aug. 22, 1972 8 Sheets-Sheet 8 MOMENTUM SPECTROME'IER The present application is a continuation in part of my application Ser. No. 15, 311 filed on Mar. 4, 1970, which was the streamlined continuation of application Ser. No. 551, 790, filed May 20, 1966, both abandoned.

This invention relates to spectrometers, and more specifically to spectrometers having a magnetic analyzer in which ions of substantially equal energy are dispersed by an amount proportional to their momentum. Such a spectrometer is known as a momentum spectrometer.

It is known for spectrometers to include one or more magnetic fields having the shape of a sector of arbitrary angle, for example a 90 sector. A beam of ions passed through a sector field is dispersed by a particular amount and is brought to a focus at a certain distance after emergence from the sector. Thus, if an attempt is made to increase the divergence by passing the beam through two sectors successively by placing the object point of the second sector field at the image point of the first sector field, it is found that the dispersion of the two fields cancel out and hence the total dispersion is equal to zero.

According to the present invention a momentum spectrometer for analyzing a beam or beams of ions of substantially equal energy according to the momentum of the ions, comprises a dispersing field which includes at least two homogeneous magnetic fields each of which has two spaced-apart parallel boundaries defining a magnetic prism, the field vectors lying in the same .direction and being arranged to be successively traversed by ions to be analyzed when the ions are introduced into the spectrometer, whereby to provide, owing to addition of the angles of deflection pertaining to the individual magnetic prisms, a resultant totat deflection of the beam, or beams, in accordance with a desired magnitude of dispersion.

There exists for each prism the relation sin a sina =blr between the entering angle a of the beam and the angle of emergence a of the beam, r being the radius of deflection of the ion pathsand b the distance between the parallel straight boundary lines of the magnetic prism.

The ion beam emerging from the dispersing field is hereinafter termed the principal beam and its central ray is termed the principal ray.

The dispersion achieved by the plurality of magnetic prisms arranged one behind the other is quite analogous to the set of prisms of a spectroscope as used in light optics which comprises a plurality of individual prisms and by means of which a greater dispersion is obtained compared with single-prism apparatus.

Unlike the case of the sector fields, when two or more magnetic prisms, that is, parallel sided fields, are connected in series, the total dispersion increases with each additional prism. If, for example, a narrow ion beam originating from an ion source and shielded by two diaphragrns is radiated into the dispersing field of the magnetic prisms and if an ion collector is arranged in the path of rays behind the dispersing field, a simple spectrometer system is provided which owing to the high dispersion is compatible with many applications even without special focusing means.

Preferably, an electric focusing field is arranged in front of the dispersing field, the electric focusing field being capable of converting a homocentric beam of rays into a beam of parallel rays and a second electric focusing field is arranged at the end of the dispersing field where the rays emerge, the second field being capable of converting said beam of parallel rays into a convergent beam.

Alternatively, magnetic focusing fields may be provided instead of the electric focusing fields described above.

In one form of the invention the arrangement of the individual prisms is such that the principal beam forms a loop and intersects in a magnetic field-free space. With such an arrangement of the path of rays it is possible to contruct a vacuum chamber, which preferably consists of a tube in the form of a closed ring, which will enclose the path of the rays and which is of a shape which can be evacuated without distortion and which occupies a relatively small space.

In a preferred embodiment of the invention, two magnetic prisms are used with a total angle of deflection of the ion beam of 180 and utilizing magnetic focusing. If, in such an arrangement, the entrance and emergence of the principal ray are chosen to take place perpendicularly to the boundary of the field both the distance of the ion source from the boundary and the distance of the collector from the boundary are chosen to be the same length as the radius of deflection, then it can be calculated that the apertural defect is equal to that of the conventional 180 spectrometer but the mass dispersion is twice as large.

In systems constructed in accordance with the present invention the number of magnetic prisms used is always greater by one than the number of magnetic field-free angular spaces provided within the entire magnetic system.

In a further embodiment of the invention, the principal beam forming a loop has a point of intersection located within a magnetic deflection field. Such a magnetic field which will be called a magnetic crossed field in the specification hereinafter and in the claims is utilized twice.

In a further embodiment of the invention, in addition to providing the arrangement described in the foregoing paragraph, the total deflection of the system is chosen so that it amounts to 360 and the principal ray prior to the deflection and the principal ray after the deflection are positioned in a straight line. This results both in a valuable increase in dispersion at a constant quality of focusing which is-determined solely by the a focusing means and in a new possibility of adjustment.

This latter feature consists in that upon switching off the magnetic field of the magnetic prisms, the ions emerging from the ion source pass directly to the collector where they may be measured by the collector for calibration purposes.

In yet another embodiment of the invention three equal magnetic prisms each having a deflection of and a crossed field which magnetic focusing are provided (FIG. 9). The radii of deflection R in the magnetic crossed field are 5/2 times greater than the radii of deflection r in the prisms and the distances between the four magnetic fields, measured on the principal ray, amount to (l/2)r. The advantageous feature of this system that the principal rays of adjacent beams of rays emerging from the system are strictly parallel to each other.

In yet another embodiment of the invention, by means of a mirror-inverted doubling of a magnet system with a deflection of 360, for example the system described in the foregoing paragraph, positive and negative ions originating from the same ion source are measurable by the same indicating apparatus with the magnetic field maintained constant. When the ion accelerating voltage varies from a positive to a negative maximum value a measuring and recording instrument intended for indicating the presence of ions will register the peaks of the positive and negative ions available in the source.

In yet another embodiment of the invention an electric radial field is arranged in front of the dispersing field for the purpose of velocity focusing. Such an arrangement involves a much smaller magnetic field with the same resolving power as the hitherto known double focusing apparatuses.

Several embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a diagram serving for the definition of a magnetic ion prism;

FIG. 1a is a section along the line a-b in FIG. 1;

FIG. 2a shows an example of a dispersing field in accordance with the present invention;

FIG. 2b shows an analogous light-optical example;

FIGS. 3a to 30 show the paths through a magnetic prism of three rays of different entrance angle;

FIG. 4 shows an arrangement of magnetic prisms with a total deflection of 180, with magnetic focusing of the first order and a radius of deflection r;

FIG. 5 shows an arrangement of magnetic prisms with a total deflection of 180, with magnetic focusing of the second order and a radius of deflection r:

FIG. 6 shows an arrangement of magnetic prisms with a total deflection of 270, with electric focusing and a radius of deflection r;

FIG. 7 shows an arrangement of magnetic prisms with a total deflection of 270, with magnetic focusing and a radius of deflection r;

FIG. 8 shows an arrangement of magnetic prisms with a total deflection of 360, with electric focusing and two radii of deflection r and R=2r;

FIG. 9 shows an arrangement of magnetic prisms with a total deflection of 360, with magnetic focusing and two radii of deflection r and R=2r;

FIG. 11 shows an arrangement of magnetic prisms which represents a doubling of the system of FIG. 9;

FIG. 12 shows an arrangement of magnetic prisms with velocity focusing, and

FIG. 13 shows an arrangement of magnetic prisms with a total deflection of 360, with magnetic focusing and two radii of deflection r and R=(1 1/4)r.

The individual Figures will now be discussed with reference to the drawings.

FIG. 1 shows a magnetic ion prism wherein B designates a homogeneous magnetic field and S an ionic ray. The prism has straight lines parallel boundaries spaced apart by a distance b; the ionic beam enters the prism with an entrance angle 01,, and is deflected along a path having ar radius r and emerging from the prism with an emergent angle a,,. The angle D represents the deflection of the beam. The entering and emergence angles, the spacing between the boundaries of the prism and the radius of the deflection path are related by the equation In each of the further FIGS. 2a, 3a, 3b, 3c, 4,5,6,7,8,9,0,11,12 and 13 crossed hatched areas bounded by parallel lines denote magnetic prisms and it will be understood that similar considerations apply to each of such magnetic prisms such as those designated by the reference numerals 30,31 and 32 in FIGS. 3a,3b, and 3c; 2 and 3 in FIG. 4; 6 and 7 in FIG. 5; 9 in FIGS. 6,9 and 11; 33,34 and 35 in FIG. 7; 9,10 and 11 in FIG. 8; 36 in FIG. 10; and 24 in FIG. 13.

FIG. 2a shows a set of prisms of an optical spectroscope analogous to the dispersing field of FIG. 2a.

FIGS. 3a, 3b and 30 show the paths through a magnetic prism of three rays of different entrance angle. In FIG. 3a, ar is equal to a and the condition of minimum deflection is achieved. In FIG. 3b, a is zero and in FIG. 3c, 0: is zero.

FIG. 4 shows a two-prism system wherein b equals k firgv designating a housing capable of being evacuated, P a passage leading to a pump, Q an ion source and Af a collector or photographic plate suitable for detecting the presence of ions. 1 and 4 designate two magnetic fields and 2 and 3 two magnetic prisms. The magnetic fields 1 and 4 are not magnetic prisms. The term magnetic prism used throughout this specification and claims is applied to fields having parallel boundaries. These parallel boundaries give rise to the effect that a parallel beam of rays of ions passing through the magnetic prism emerges as a parallel beam. This effect does not occur in the case of the magnetic fields 1 and 4.

FIG. 5 shows a two-prism system wherein P equals 3516A and b equals et 5 r. s and s designating two magnetic fields and 6 and 7 two magnetic prisms.

FIG. 6 shows a three prism system in which the rays enter the prism 9 in the condition for minimum deflection as in FIG. 3a and wherein q equals and b equals. 1/7 r, K designating a point of intersection.

FIG. 7 shows an arrangement of three prisms 9a having two field-free spaces between them each of which includes an angle of b equals 2r. sin 52.5 and K designates the point of intersection. The coefficient of dispersion D is defined by the equation D =dp dr wherein dr is the difierence of the radii of deflection of two irradiated kinds of ions of the masses m and (m+dm), the ions of the mass m travelling along the principal ray, and dp is the distance of the focusing point of the ion beam (m+dm) from the principal ray. For conventional symmetrical sector field systems of 60, 90 and 180 d is equal to 2. D for the system of FIG. 7 is 8.35 which is 4.2 times greater than that of the conventional systems.

FIG. 8 shows a system comprising three magnetic prisms 9 each having a deflection of 90 and a crossed field. Focusing is achieved by electric lenses I2 and 13. Two embodiments 10 and 11 of crossed field are indicated, the embodiment 1 1 achieving a greater dispersion than the embodiment 10. The radii of deflection R in the magnetic crossed field are two times greater than the radii of deflection 2 in the prisms and b= 1F 2 14 designates a principal beam and 15 a principal ray. There are five prisms in all, and therefore there are four field-free spaces, the crossed field being passed through twice.

FIG. 9 shows an arrangement of three prisms 9 as in FIGS. 6 and 8 each having a deflection of 90 with a crossed field 16. On either side of an emerging principal beam 14 comprising ions of mass number 80 there are drawn substantially to scale two neighboring beams 17 and 18 comprising ions of the mass numbers 79 and 81. D is equal to 2 (3 V 2+1)=l0.5, i.e., the mass dispersion is 5.2 times greater than that of a conventional system with a radius of deflection R.

The radii of deflection R in the magnetic crossed field are 5/2 times greater than the radii of deflection r in the prisms 9 and b= fir.

FIG. 10 shows a system comprising two magnetic prisms 36 of 120 deflection and a magnetic crossed field, the radii of deflection R in the cross-field being twice the radii of deflection 2 in the prisms 36 b= fir.

FIG. 11 shows a seven-part system formed by a mirror-inverted doubling of the system of FIG. 9 and serving for recording positive and negative ions in one single spectrum. G designates a gas intake, Q the ion source, P a passage leading to a pump, V a vacuum vessel and Af the collector. 9 designates individual prisms having deflections of 90 and 19 a crossed field of double construction, the right-hand loop drawn in full lines representing the path of the positive ions and the lefthand loop drawn in broken lines representing the path of the negative ions.

FIG. 12 shows schematically an electric radial field 20 which, for the purpose of velocity focusing, is ar ranged in front of a magnetic 360 deflecting system of which only a crossed field 22 is shown in the Figure. 19a designates input diaphragms, 21 an energy diaphragm and 23 the point of velocity and direction focusing.

FIG. 13 shows a five-part system with a deflection of 360. It consists of four prisms 24 each having a deflection of 72 and of a crossed field 25 with magnetic focusing. The system has five field-free and hence six prisms. The radii of deflection in the magnetic crossfield are 11/4 times greater than the radii of deflection Z in the prisms.

The above-described series of systems 3Xl20 (FIG. 10), 4X90 (FIGS. 8,9 and 11), 5X72 (FIG. 13) may be continued by a system 6X60 etc.

What I claim is:

1. A momentum spectrometer for analyzing a beam or beams of ions of substantially equal energy according to the momentum of the ions, comprising means for producing an ion dispersing field which includes at least two magnetic elements for producing homogeneous fields having field vectors in the same direction,

each of said magnetic elements having two spaced apart parallel boundariesdefining a magnetic prism and ion entrance and exit faces, said magnetic elements being arranged with respect to said ion entrance and exit faces to be successively traversed by ions to be analyzed when the ions are introduced into the spectrometer and to be symmetrical on opposite sides of an imaginary plane through the center of the spectrometer and perpendicular to an ion beam or beams passing therethrough, whereby to provide, owing to the addition of the angles of deflection pertaining to the individual magnetic prisms, a resultant total deflection of the beam or beams in accordance with a desired magnitude of dispersion according to momentum; a first focusing means for converting the input beam of ions on the object side of said spectrometer into a beam of parallel rays, which is transmitted into said dispersing field producing means for detecting the presence of ions and second focusing means on the image side of said spectrometer for receiving parallel beams of rays emitted from the means for producing a dispersing field that have been separated according to momentum and focusing each of the respective separated beams onto a means for detecting the presence of ions.

2. A spectrometer as claimed in claim 1, in which the said individual prisms are positioned to cause a principal beam to form a loop and to intersect in a magnetic field-free region.

3. A spectrometer as claimed in claim 2, in which the respective parallel sided boundaries of the prisms make equal angles with said beam, and the angle of deflection of the dispersing field plus the angle of deflection of the focusing fields, amounting to 180.

4. A spectrometer as claimed in claim 2, in which there is provided a crossed magnetic field, said magnetic prisms being arranged for a beam passing therethrough to loop and to intersect said crossed magnetic field, the crossed magnetic field thus being used twice in respect of its dispersing action owing to twice repeated passage of the beam of rays.

5. A spectrometer as claimed in claim 4, wherein said loop is 360.

6. A spectrometer as claimed in claim 5, wherein the means for forming the dispersing field has three equal magnetic prisms with a radius of deflection r and a width b= fir, each prism having a minimum angle of deflection said means for forming said crossed magnetic field being constructed as a magnetic prism of a width 2r and the radius of deflection in said crossed magnetic field R being twice as large as in the magnetic prism, whereby the principal rays enter and emerge from the dispersing field perpendicular to the boundary of the field and wherein said focusing means comprises electric field producing means.

7. A spectrometer as claimed in claim 5, wherein the means for forming the dispersing field comprises three equal magnetic prisms with a radius of deflection r and a width b= fir, each prism having a minimum angle of deflection (I 90 and wherein the crossed magnetic field is constructed as a magnetic focusing field capable of focusing in the first order and having a radius of deflection R=(5/2)r, the principal ray entering and emerging perpendicularly to the boundary of the crossed field, and wherein the distances of the focusing points from the boundary of the field (5/2)r and the four distances of the four magnetic fields between each other, measured on the principal ray, amount to (%)r.

3. A spectrometer as claimed in claim 5, wherein the means for forming the dispersing field has two equal magnetic prisms with a radius of deflection r and a width b 1/ 3r, each prism having a minimum angle of deflection l 9. A spectrometer as claimed in claim 5, wherein an indentical, magnetic field arrangement is disposed in a it mirror-inverted manner symmetrical about the straight line on which the principal ray entering the crossed field perpendicularly to the boundary thereof and the principal ray emerging from the crossed field perpendicularly to the boundary thereof are jointly located such that positive and negative ions can be projected into the crossed field and measured by means of the equal magnetic prisms with a radius of deflection r and each having a minimum angle of deflection 1 72 and wherein said crossed magnetic field deflects a beam 36 in each direction with a radius of deflection R which is greater than r. 

1. A momentum spectrometer for analyzing a beam or beams of ions of substantially equal energy according to the momentum of the ions, comprising means for producing an ion dispersing field which includes at least two magnetic elements for producing homogeneous fields having field vectors in the same direction, each of said magnetic elements having two spaced apart parallel boundaries defining a magnetic prism and ion entrance and exit faces, said magnetic elements being arranged with respect to said ion entrance and exit faces to be successively traversed by ions to be analyzed when the ions are introduced into the spectrometer and to be symmetrical on opposite sides of an imaginary plane through the center of the spectrometer and perpendicular to an ion beam or beams passing therethrough, whereby to provide, owing to the addition of the angles of deflection pertaining to the individual magnetic prisms, a resultant total deflection of the beam or beams in accordance with a desired magnitude of dispersion according to momentum; a first focusing means for converting the input beam of ions on the object side of said spectrometer into a beam of parallel rays, which is transmitted into said dispersing field producing means for detecting the presence of ions and second focusing means on the image side of said spectrometer for receiving parallel beams of rays emitted from the means for producing a dispersing field that have been separated according to momentum and focusing each of the respective separated beams onto a means for detecting the presence of ions.
 2. A spectrometer as claimed in claim 1, in which the said individual prisms are positioned to cause a principal beam to form a loop and to intersect in a magnetic field-free region.
 3. A spectrometer as claimed in claim 2, in Which the respective parallel sided boundaries of the prisms make equal angles with said beam, and the angle of deflection of the dispersing field plus the angle of deflection of the focusing fields, amounting to 180*.
 4. A spectrometer as claimed in claim 2, in which there is provided a crossed magnetic field, said magnetic prisms being arranged for a beam passing therethrough to loop and to intersect said crossed magnetic field, the crossed magnetic field thus being used twice in respect of its dispersing action owing to twice repeated passage of the beam of rays.
 5. A spectrometer as claimed in claim 4, wherein said loop is 360*.
 6. A spectrometer as claimed in claim 5, wherein the means for forming the dispersing field has three equal magnetic prisms with a radius of deflection r and a width b Square Root 2r, each prism having a minimum angle of deflection Phi 90*, said means for forming said crossed magnetic field being constructed as a magnetic prism of a width Square Root 2r and the radius of deflection in said crossed magnetic field R being twice as large as in the magnetic prism, whereby the principal rays enter and emerge from the dispersing field perpendicular to the boundary of the field and wherein said focusing means comprises electric field producing means.
 7. A spectrometer as claimed in claim 5, wherein the means for forming the dispersing field comprises three equal magnetic prisms with a radius of deflection r and a width b Square Root 2r, each prism having a minimum angle of deflection Phi 90* and wherein the crossed magnetic field is constructed as a magnetic focusing field capable of focusing in the first order and having a radius of deflection R (5/2)r, the principal ray entering and emerging perpendicularly to the boundary of the crossed field, and wherein the distances of the focusing points from the boundary of the field (5/2)r and the four distances of the four magnetic fields between each other, measured on the principal ray, amount to ( 1/2 )r.
 8. A spectrometer as claimed in claim 5, wherein the means for forming the dispersing field has two equal magnetic prisms with a radius of deflection r and a width b Square Root 3r, each prism having a minimum angle of deflection Phi 120*.
 9. A spectrometer as claimed in claim 5, wherein an indentical, magnetic field arrangement is disposed in a mirror-inverted manner symmetrical about the straight line on which the principal ray entering the crossed field perpendicularly to the boundary thereof and the principal ray emerging from the crossed field perpendicularly to the boundary thereof are jointly located such that positive and negative ions can be projected into the crossed field and measured by means of the same collector without a reversal of the polarity of the dispersing field.
 10. A spectrometer as claimed in claim 1, wherein said first focusing means produces an electric radial field.
 11. A spectrometer as claimed in claim 1, wherein said means for forming the dispersing field has four equal magnetic prisms with a radius of deflection r and each having a minimum angle of deflection Phi 72* and wherein said crossed magnetic field deflects a beam 36* in each direction with a radius of deflection R which is greater than r. 